Method of making a surgical laser fiber from a monolithic silica titania glass rod

ABSTRACT

A new optical fiber and method of manufacturing the same developed for use with surgical laser systems. The fiber core utilizes an ultra-low expansion (ULE) material. The preferred ULE fiber consists of silicon dioxide core doped with titanium dioxide which is cladded and jacketed for chemical and abrasion resistance. The resulting fiber is stable against degradation due to thermal expansion.

STATEMENT REGARDING GOVERNMENT RIGHTS

The present invention was made with government support from the U.S.Department of Commerce under Grant No. ITA 87-02. The Government mayhave certain rights in this invention.

RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 07/950,913 filed on Sept. 24, 1992 now abandoned,which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the field of laser fibers. Moreparticularly, the present invention relates to laser fibers constructedof ultra-low-expansion (ULE) materials and methods of making suchfibers.

BACKGROUND OF THE INVENTION

(Throughout this Specification, there appear numbered superscripts.These refer to prior art references listed by number in the "OtherDocuments" portion of the "Information Disclosure Statement" thataccompanied the parent application identified above.)

For many years, lasers have been used for a variety of surgicalprocedures. The desired effects may be thermal, photodisruptive orphoto-chemical. Lasers may be delivered to and targeted on the treatmentsite by either a free beam system using mirrors or by directing thelaser light in an optical fiber. Optical fibers have been the preferredsystem of laser delivery for surgical procedures, if the desiredwavelength can be transmitted through the fiber. In a non-contact mode,the optical fiber has no effect on the wave-length specific tissueeffect.

Most lasers currently used for medical-surgical procedures are used fortheir thermal effect which is dependent upon the specific way the laserlight energy is transferred to thermal energy in the tissue. A commonlaser used is the carbon dioxide (CO₂) laser whose wavelength at 10.6 μmmakes it opaque to cellular water. The CO₂ laser is therefore totallyabsorbed by water and rapidly converted to thermal energy over a veryshort distance (<100 μm).

Recently, Nd:YAG laser systems, coupled to silica fibers with eithersculptured or bare tips or with sculptured sapphire tips, have showngreat benefits as surgical tools when used for certain procedures.¹⁻⁶Using these laser systems with a bare fiber, photocoagulation to tissuedepths of 4 to 5 mm in tissue can be attained in a non-contact mode.⁶⁻¹²In a contact mode, incision and cauterization of the nearby tissue canbe achieved.¹³⁻²⁰

These two capabilities, though providing the surgeon with new andpowerful tools in performing procedures that are very close to beinghemostatic, have as yet to be integrated into a full spectrum of laseroptical fiber surgical systems. At one extreme, only photocoagulationcan be achieved in a non-contact mode while at the other extreme onlyincision can be achieved in a contact mode. Between these two extremes,there is a range of combined and controlled photocoagulation andincision that would be highly desirable, and a fiber optic system thatcould provide this full-spectrum capability would provide the surgeonwith a broad range of new surgical capabilities to meet the specificneeds of a broad range of surgical procedures.

The most common optical fiber material used for the delivery of laserlight energy at the present time is silica glass. Indeed, the same glasschemistry presently used in all optical fibers for laser surgery is alsoused in telecommunication optical fibers. These optical fiber cables arecapable of transmitting laser light through very small diameters andthey can be threaded to almost any part of the body creating little orno damage to surrounding normal tissue. These fiber optic deliverysystems are, therefore, ideal for use with endoscopy.

Such silica fibers have been shown to be highly effective inphotocoagulating tissue in a non-contact mode. However, when thesefibers come into contact with tissue or blood there is significantthermal-mechanical damage to the fiber and disruption of lighttransmission. This disruption limits the focusing of the laser andcauses the laser energy incident upon the damaged optical fiber tip tobe converted into thermal heating of the tip. Hence, after thedegradation of the optical fiber tip has begun, the fiber can no longerbe used for photocoagulation.

When the fiber optic cable contacts tissue, there is a marked change inthe tissue response and a significant reduction in the tissuetransmission of the laser light. Because of the absorption of laserenergy at the optical fiber tip at the tissue contact surface, the fiberis rapidly heated to high temperatures thereby destroying the opticalquartz fiber. The effect on the surrounding tissue is variable and hasan unpredictable tissue damage pattern.

The thermal-mechanical breakdown of the optical fiber that follows theuse of the fiber in a contact mode also results in the contamination ofthe incisional site with silica glass fragments. These fragments maypresent a bio-hazard as their effect on tissue has not been fullystudied. Perhaps more importantly, the use of a technique for preciselaparoscopic dissection that creates a variable tissue effect withsignificant lateral coagulation is less than optimal.

Sapphire tips have been used in an attempt to incise tissue withoutdestruction of the optical fiber. Examples of this approach are found inDaikuzono U.S. Pat. No. 4,693,244 and Hoshino U.S. Pat. No. 4,832,979.

Sculptured sapphire tips range in shapes from long small-angle cones, toshorter sharper-angle cones, to blunt end cylinders to roundedsemi-spheres. It is believed that by changing the angle at the end ofthe sapphire tip, the focusing or defocusing of the laser light can beachieved; small, sharp angles tending to focus the light down to smallerangles whereas larger angles, such as the rounded semi-sphere ends,tending to defocus the laser light. The belief is that by changing thisangle, the nature of the tissue response can be changed from incision atthe highly focused extreme to photocoagulation at the defocused extreme.

Although this effect of focusing and defocusing may be the case when theoptical fiber is used in non-contact, it is likely not the case whenused in the contact mode. Rather it is our experience that with thesapphire tips, the energy transmission from the fiber to the tissue isdominated by the contact surface between the tissue and the sapphiretip. As with the optical fiber, for a short time, the sapphire tip willlase in contact with the tissue but then it breaks down and the tip willonly incise the tissue in a manner similar to that of the silica fibertip. The sapphire tip, however, due to its single crystal structure,when heated by the laser fails catastrophically along well-definedcrystallographic planes. Hence, rather than small pieces or flakes ofthe sapphire tip coming off the tip in a gradual manner as is the casewith the silica optical fiber, the sapphire tip cracks along the entirelength of the tip and large chunks of the tip fall off into thesurrounding tissue area. Such failure is catastrophic in the sense thatthese larger pieces of sapphire (typically, 1-2 mm×1-2 mm) are quite hotdue to the laser heating and when they contact the tissue, they causethermal damage to surrounding normal tissue in the surgical area andmust be retrieved from the site.

Another failure mechanism of fibers with sapphire tips is that becausethey are composite systems, the sapphire tip must be connected to theoptical fiber cable. The most common method of attachment includes asmall brass ferrule compression fitted onto the teflon protective jacketof the fiber. Because the sapphire is a good thermal conductor, thebrass ferrule is heated along with the sapphire tip and, in someinstances of continued use, melts the teflon jacket and falls off intothe surgical area. In this case, both the sapphire tip and the brassferrule can burn the tissue at the surgical site. As a result, both thesapphire tip and brass ferrule must be retrieved from the surgery sitebefore the damage to the tissue is extensive. Hence, as with the silicafibers, the sapphire tips degrade and contaminate the surgery tissuesite when used in a contact mode.

Other attempts at providing fibers for contact laser surgery have beendirected to coating the fiber tip with infrared absorbing material, astaught in Daikuzono U.S. Pat. No. 4,736,743, or, in a manner similar tothat used in sapphire contact tips, changing the tip configuration tomodify the laser beam and reduce localized heating at the tip. Theseattempts have not met with success because, even though they arespecifically designed for contact applications, they suffer from thesame thermal expansion and eventual degradation described above for thesilica fiber tips and sapphire tipped fibers.

The use of laser energy in the blue-green region of the electromagneticspectrum to incise tissue provides advantages in tissue with a highhemoglobin concentration (usually indicated in tissue with high degreesof vascularity such as the liver). The absorption of such energy isincreased by the high iron content of such regions. These systems alsouse simple and inexpensive silica optical fiber delivery system. Oneimportant disadvantage is that these systems are typically used only innon-contact procedures. Presumably, these silica-based laser systemswould suffer the same thermal-mechanical break down as do the Nd:YAGlaser fiber systems if used in contact procedures.

Although some of the fibers described above can provide adequateperformance when used in non-contact applications to accomplishphotocoagulation, the reality facing surgeons is that contact betweenthe fiber tips and tissue is extremely difficult to avoid due to theclose quarters in which these devices operate. Once the fiber orsapphire tip contacts tissue and degradation of the tip surface occurs.After the initial degradation occurs, the performance (for eitherphotocoagulation or incision) of the fiber can no longer be accuratelypredicted by the surgeon. If predictable characteristics are required, anew fiber tip must be created, either by cutting and polishing a new tip(a laborious, time consuming process) or by replacing the entire fiber.Both of these options increase the cost of the laser surgery.

In summary, the laser optical fiber systems currently in use, silica orotherwise, remain useful as photocoagulating or incisional tools only solong as they are used in non-contact procedures. Given the close natureof the environments in which these devices operate, however, contact andthe resulting degradation are difficult, if not impossible, to avoid. Assuch, there is a need for a fiber material which provides predictableoperating characteristics in either a contact or non-contact mode andwhich can be modified to provide a predictable balance betweenphotocoagulation and contact incision.

SUMMARY OF THE INVENTION

The present invention provides a fiber optic cable with a core formed ofULE materials which exhibits predictable characteristics regardless ofits use in methods involving non-contact, contact or a combination ofboth. As a result, fibers according to the present invention are usefulfor photocoagulation, incision (contact or non-contact) or a combinationof photocoagulation and incision.

Because of the extremely low expansion coefficients of the ULEmaterials, fibers according to the present invention retain theirintegrity and, as a result, their characteristics such as energytransmission/absorbance far longer than fibers constructed ofconventional materials.

Furthermore, because the ULE material does not degrade rapidly whenheated, it can be intentionally processed to absorb a predeterminedpercentage of laser energy at the tip, thus providing a useful thermalincision device, in addition to providing a consistent, predeterminedpercentage of laser energy for photocoagulation. The energy absorptionis preferably accomplished by doping the fiber tip with an absorbingspecie, such as a colored transition metal ion which provides a desiredlevel of energy absorption. That predetermined level of energyabsorbance can be used to control the extent of the input laser energythat is used to incise tissue. If no dopant were added to the tip, thensubstantially all of the laser energy incident on the tip would betransmitted to the tissue and complete photocoagulation would result. Ifthe tip were doped with a laser absorbing specie then a fraction oflaser energy would be absorbed at the fiber contact tip and the tipwould be heated to a temperature that would be proportional to the levelof doping in the tip. Lightly doped tips will incise tissue lightly andheavily doped tips will incise tissue heavily. In the extreme of doping,substantially all of the laser energy would be absorbed at the fiber tipand the result would be a 100% incisional fiber tip.

Although fully incisional fiber tips already exist (i.e., those fiberswhich have suffered thermal-mechanical breakdown either by using tissueor the commonly used practice of degrading the fiber with a woodentongue blade), the benefit with the fiber system of the invention isthat a fully incisional fiber would be but one of a full spectrum ofsuch fibers where the extent of photocoagulation or incision ispredetermined. Furthermore, the silica fibers in common use must beintentionally damaged to yield this performance and this damagecontinues so long as the fibers are used, leaving behind a small butmeasurable residue of glass fragments in and along the tissue incisionalsite.

In one aspect of the invention, the laser optical fiber comprisesultra-low expansion (ULE) materials, coated with a waveguiding claddingmaterial, typically a UV-curable silicone acrylate polymer, jacketedwith a protective polymer, typically teflon, and used with a laser,typically a Nd:YAG laser. Such optical fibers exhibit near zero thermalshock over a wide range of temperatures and as a result remain stablewhen used in contact with tissue and energized with a laser.

ULE fibers according to the present invention can also be doped orotherwise modified to provide a range of controlled photocoagulationand/or incision of tissue depending upon, for example, the extent ofdoping with a laser absorbing specie, typically a transition metal ion,like Fe⁺² or Fe⁺³.

These and various other advantages and features which characterize theinvention are pointed out with particularity in the attached claims. Fora better understanding of the invention and its advantages, referenceshould be made to the drawings which form a part hereof, and to theaccompanying description, in which various preferred and alternateembodiments and methods according to the present invention are describedin more detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart showing the percent of linear expansion versustemperature of four different bulk material and fiber specimens;

FIG. 2 is a chart showing the fractional linear expansion versustemperature for a series of titania (TiO₂) doped silica (SiO₂) fibers;

FIG. 3 is a chart showing the average expansion coefficients, defined asthe rate of change of the linear expansion with temperature, for threeranges of temperatures, for titania doped silica fibers of differentweight percentages; and

FIG. 4 depicts the relative relationship between fibers according to thepresent invention and known silica glass fibers when compared on a powervs. time graph indicating stable working regions.

FIG. 5 is a chart showing the range of performance of an optical fiberutilizing the principles of the invention.

FIG. 6 is a flowchart depicting one method of manufacturing fibersaccording to the present invention.

DESCRIPTION OF THE PREFERRED AND ALTERNATE EMBODIMENTS OF THE INVENTION

According to the principles of the invention, the problem of the opticalfiber thermal-mechanically degrading when in contact with tissue iscontrolled by increasing the thermal shock resistance of the opticalfiber. To accomplish this, ULE material fibers are used which, whencoated with a hard polymer cladding and jacketed with a tough teflonabrasion and chemical resistant layer, provide a surgical optical fiberthat is resistant to thermal shock-induced failure.

Typically, known silica fiber core materials exhibit thermalcoefficients of linear expansion of 0.5×10⁻⁶ /°C. or greater for workingtemperatures of 25° to 700° C. Preferred ULE materials according to thepresent invention, however, exhibit thermal coefficients of linearexpansion of less than about 0.5×10⁻⁶ /°C. in the same temperaturerange. The exact coefficients of the ULE materials are, of course,dependent on temperature and the nature of the ULE material being used.At all temperatures, however, ULE materials according to the presentinvention will exhibit linear thermal coefficients of expansion lessthan that of silica glass.

The preferred ULE material according to the present invention comprisestitania-doped silica glass, variations of which are described in greaterdetail below.

FIGS. 1 and 2 show that silica fibers exhibit substantially largerthermal expansion coefficients than titania-doped ULE silica glasses.FIG. 1 is a chart illustrating the linear expansion of four differentbulk glasses and fibers at various temperatures: a silica (quartz) glassrod, a section of a commercial 600 μm silica optical fiber, atitania-doped silica ULE glass rod and a section of ULE ˜600 μm opticalfiber.

FIG. 1 shows that the ULE glass exhibits a much less expansion than thatof the commercial silica. FIG. 1 also shows that the ULE glass exhibitsthe ULE behavior even in the fiber form. Because thermal stresses acrossa given fiber generated by a temperature gradient are inverselyproportional to the expansion coefficient of the material used to formthe fiber, it is expected that such ULE fibers would be much lesssusceptible to thermal stress failure and as a result tend to remainstable when the fiber is brought into contact with tissue and energizedwith a laser.

FIG. 2 shows that the thermal expansion of the titania-doped glassdecreases substantially as the titania dopant concentration increasesfrom 0 to about 6-8% (weight). FIG. 2 also shows that the titania dopingdramatically decreases the linear expansion to the extent that by 6 to7.5 wt % TiO₂, the expansion coefficient is near zero for a wide rangeof temperatures, up to ˜800° C. The slope of each curve shown on FIG. 2is defined as the expansion coefficient, which determines the thermalshock resistance of the fiber.

These slopes are shown on FIG. 3 for three ranges of temperatures, +5°to +35° C., +25° to +700° C., and -200° to +5° C. In the presentapplication, the range of +25° to +700° C. is applicable. FIG. 3 showsthat for this range the average expansion coefficient reaches zero(infinite thermal shock resistance) near a doping of 7.5 wt % TiO₂.Thus, a glass with this doping would have a thermal shock resistanceapproaching infinity. Although in practice such values would never beattained, the thermal shock resistance would increase dramatically forglass with a titania-doping level near 7.5 wt %

FIG. 4 is a graphic representation of the relative stable workingregions (as a function of laser power versus time) of both fibersaccording to the present invention and known fibers. The working regionsof fibers according to the present invention, represented by the areaunder line 10, provide an increased stable working envelope whencompared to the area under line 12, which represents the stable workingregion of known fibers. As shown in FIG. 4, ULE fibers can be used forlonger periods of time and/or at higher power levels (relative to silicaglass fibers) without degradation sufficient to substantially affect theperformance of the fibers.

It should be noted that FIG. 4 depicts the relationship between lines 10and 12 in a relative, as opposed to absolute sense. The actualrelationship will depend on the characteristics of the tissue in contactwith the fibers and other factors. In all cases, however, the stableworking limits of ULE fibers according to the present invention willexceed those of known silica fibers.

Because of the stable nature of ULE fibers according to the presentinvention, they may be intentionally provided in embodiments whichabsorb a desired, predetermined amount of laser energy which is thenconverted to heat for incision or other purposes. Known silica fiberswould degrade under such conditions, but the low linear thermalcoefficients of expansion of ULE materials used in fibers according tothe present invention, provide a larger range of thermal stability.Techniques of modifying the fibers to include a means for heating thefiber tip through the absorption of laser energy include, but are notlimited to, abrading the fiber tip or doping the tip with a laserabsorbing specie.

The preferred method of modifying the ULE fibers to increase energyabsorption and decrease transmission of laser energy is doping the fibertips. The fiber tips can be doped to varying extents with a laserabsorbing specie to provide for controlled photocoagulation and/orincision of tissue depending upon the extent of doping of the fiber tip.Fibers which are more heavily doped will absorb more laser energy,converting it into thermal energy at the tip of the fiber. Fibers whichare lightly doped, or not doped at all, will absorb less laser energy,thereby passing more energy out of the fiber to provide increasedphotocoagulation.

Preferably, the laser absorbing specie is a transition metal-ion likeFe⁺² or Fe⁺³. Other laser absorbing specie can be used in place of thepreferred specie. Examples include, but are not limited to: Co⁺², Mn⁺²and Ni⁺².

The preferred method of doping the tip with the laser absorbing specieis to coat the area to be doped, typically the distal end tip, with asolution or slurry containing the dopant. The tip and solution are thenheated to about 1000° C. in a small electric furnace. By holding thecoated tip at this temperature for a period of time, preferably a fewminutes to less than an hour, the dopant will dissolve into the tipcausing a homogenous concentration throughout the tip volume. Theconcentration in the tip is governed by the amount and concentration ofdopant placed on the surface of the tip prior to heating.

Alternate methods of doping the ULE fibers according to the presentinvention will be known to those skilled in the art, including, but notlimited to ion implantation, etc.

FIG. 5 illustrates the range of the performance of various opticalfibers produced according to the principles of the invention, which maybe used in contact with tissue. At the far left, FIG. 5 depicts theperformance of a fiber tip that has no laser absorbing dopant and, as aresult, the fiber transmits 100% of the laser energy into the tissuemedium. At the far right, FIG. 5 depicts the performance of a fiber tipdoped with the maximum amount of the laser absorbing dopant and as aresult, the fiber converts 100% of the laser energy into heat at thefiber tip.

Between the two extremes depicted in FIG. 5, a range of dopantconcentrations exist where a fraction of the laser energy is convertedto heat at the tip and the remaining fraction of laser energy istransmitted through the fiber tip to the time. This characteristicenables the production of a range of different laser optical fiberswhich can be used to provide a predetermined amount of bothphotocoagulation and incision depending upon the amount of doping.

In addition to being able to withstand the natural thermal cycling thatresults from energizing these optical fibers in contact with tissue, ULEoptical fibers also have the unique ability to withstand the thermalcycling arising from high peak energy levels in use with rapidly pulsed"Q-switched" laser systems. Whether thermal cycling arises from thetissue contact or from internal heating due to a pulsed laser or acombination of the two or other phenomena, the near zero expansion ofthe ULE optical fibers makes them uniquely suited for such applications,especially new applications such as photo-disruptive laser therapyand/or surgery.

Such titania-doped silica fibers also are non-toxic, straight forwardand inexpensive to prepare, chemically inert, and are opticallytransparent from the ultraviolet to the infrared.

For these reasons, ULE optical fibers can be produced and fiberized intocontact tips for use with all standard laser optical fiber deliverysystems. Such fibers would be compatible with all medical laser opticalfiber delivery systems using visible and near-infrared laser energy. Thediameters of such fibers typically range from 100 to 1000 μm, with themost common being 600 μm. These diameters are the most commonly useddiameters in practice today. The optical connectors for the tips arestandard "SMA" connectors commonly used in laser systems. The fibers canbe attached into fiber handholding devices in a fashion commonly in usetoday. In short, all of the known uses, optical connectors, laserdelivery systems and packaging methods are completely compatible withthe ULE optical fibers of the invention. In fact, this compatibility isa tremendous advantage.

As is standard in the art and practice and known to those skilled in theart, fibers produced using the principles of the invention are clad witha hard polymer and jacketed with a tough buffer material, typicallyteflon or tefzel. The preferred cladding is a fluoracrylate material,although other cladding materials can also be used, including, but notlimited to silicone acrylate polymers and other materials.

The distal ends of the fibers can be formed into any of a range ofdifferent configurations. All such configurations and others commonlyused are suitable for use with the ULE optical fibers of the inventionwhen used either in a contact or non-contact mode.

All configurations of the ULE laser optical fibers of the invention arecompatible with Nd:YAG, argon-ion, KTP or any other laser systemoperating between the wavelengths of ≈200 nm to 3000 nm. The preferredULE laser optical fibers of the invention are compatible with theselasers producing up to ≈120 Watts for a fiber diameter of 600 μm.

Although the new ULE laser optical fiber of the invention has beendeveloped to be 100% compatible in every way to the pure silica laseroptical fiber, it has been found that differences do exist in the way inwhich the ULE optical fibers are prepared.

In a typical preparation of silica optical fibers, a prepolished puresilica fiber preform glass rod is chucked into a preform holder,inserted into a graphite element argon gas environment furnace, heatedup to the fiber drawing temperature, and then pulled into fiber form ata precisely controlled speed and temperature. The pulled fiber is thenimmediately coated with a hard acrylic protective and wave-guidingcladding, tension tested for minimum strength and then wound onto a takeup spool. Such fiber drawing practices have been developed over sometime, and the ULE fiber drawings is compatible with all of thesepractices except that the fiber is preferably pulled using an air oroxygen atmosphere furnace. Referring to FIG. 6, one process ofmanufacturing a fiber according to the present invention is depicted.

The air or oxygen atmosphere is required to keep the titanium in thedoped preform in its highest (Ti⁺⁴) oxidation state. If the glasspreform is reduced (Ti⁺⁴ →Ti+³) then this reduces the fibers' ability totransmit the laser light because of the blue coloring indicative of theTi+³ in the fibers. Such fibers cannot be used as surgical tools becausethey will not transmit laser energy.

It has been our finding that the commercially available ULEtitania-doped silica glass contains reduced titania (Ti+³) such thatwhen core-drilled preforms of this glass are pulled, even in oxidizingatmospheres, the fibers are severely colored blue and will not transmitlaser energy. This problem makes commercially available ULE glasspreforms unusable for fiberizing laser optical fibers.

For this reason, the glass preform must be processed to the proper Ti+⁴oxidation state before it is pulled into a fiber useable fortransmission of laser energy according to the present invention. In onepreferred method, annealing the glass preform in air at 1000° C. forperiods ranging from 18 to 24 hours, the glass preforms will oxidize tothe Ti⁺⁴ state such that when the fibers are pulled from these annealedpreforms they are colorless and will transmit laser light. In analternate preferred method, the preforms may be annealed for 48-72 hoursin an oxygen atmosphere at temperatures of 1000° to 1200° C. beforepulling. ULE fibers pulled from preforms treated according to thepresent invention transmit substantially all of the laser energydirected through them.

Apparently, the Ti⁺³ in the commercially available titania-doped ULEmaterial arises from the manufacturing process, and therefore changes inthe process could result in preforms already in the oxidized state.Currently, chemical vapor deposition (CVD) techniques are used toprepare the glass preforms. One such CVD technique is taught in U.S.Pat. No. 2,326,059. These prior art processes do not take specialprecautions to insure the correct oxidization state of the titanium inthe glass and, as a result, typically could not be used to manufactureuseable laser fibers. Special precautions are taken only to insure thatthe titanium is delivered to the reaction chamber in high purity and inthe correct stoichiometry. It is believed that the glass preformsproduced by known CVD techniques result in substantial amounts ofreduced titania, i.e., Ti⁺³. Therefore, to produce glass preforms thatcan be pulled into fibers suitable for transmission of laser energy,special care must be taken to insure not only high purity and correctstoichiometry, but the correct oxidation state of the titanium as well.

In the methods according to the present invention, pure oxygen is usedin the CVD reaction chamber resulting in the oxidation of the TiCl₄ andSiCl₄ gas streams to TiO₂ and SiO₂, respectively, thus maintainingsuitable levels of Ti⁺³ and Ti⁺⁴ in the ULE fibers sufficient to allowthe transmission of substantially all laser energy directed through thefibers.

Fibers thus produced according to the principles of the inventionprovide a safe and reliable means of photocoagulating tissue with alaser activated optical fiber without exhibiting the thermal-mechanicalbreakdown phenomenon common to ordinary silica-based fibers. The fibersof the invention will provide a surgeon using lasers with a stable fiberwhose properties and performance do not change throughout a typicallaser surgical procedure. The increased stability of the optical fiberof the invention will therefore enable surgeons to perform laser surgeryprocedures more reliably, controllably and safely than when ordinarysilica fibers are used.

The physical, chemical, and optical properties of the titania-dopedsilica optical fibers of the invention are similar to those of thesilica fibers except that they exhibit a slightly higher index ofrefraction and, as described above, a much lower, near zero, linearcoefficient of thermal expansion. The similarities allow them to be usedwith all the common lasers used in surgery.

The preferred fibers of the invention can also be provided in the samediameters and lengths and use the same optical and surgical connectorsand fittings as those of the silica fibers which, combined with thesimilar optical properties, will allow them to be used completelyinterchangeably with all the common commercial surgical lasers now beingused with the silica-based fibers. The laser-optical fiber-tissueresponse is also identical to that resulting from the silica-basedoptical fiber system, except that the titania-doped silica fiber remainsstable throughout the surgical procedure. The cost and technicalrequirements of producing the titania-doped optical fibers of theinvention are also very similar to those of the silica fibers. All ofthese similarities to the silica-based optical fibers of the invention agreat improvement in the optical fiber laser surgery technology.

Having thus described the invention in connection with the preferredembodiment thereof, it will be evident to those skilled in the art thatvarious revisions and modifications can be made to the preferredembodiment without departing from the spirit and scope of the invention.It is our intention, however, that all such revisions and modificationsas are obvious to those skilled in the art will be included within thescope of the following claims.

What is claimed is:
 1. A method of manufacturing a laser fibercomprising the steps of:a) providing a preform rod consistingessentially of silica-titania glass exhibiting a coefficient of linearthermal expansion of less than about 0.5×10⁻⁶ /°C. in temperatures fromabout 25° C. to about 700° C.; b) heating the preform rod to a drawingtemperature; c) pulling the rod to form an optical fiber; d) forming atip on the fiber; and e) doping the fiber tip with a laser absorbingdopant.
 2. The method of claim 1, wherein the step of doping furthercomprises doping the fiber tip with a predetermined concentration of thedopant.
 3. A method according to claim 1, wherein the preform rodcomprises a concentration of TiO₂ in a range of about 6 to 8 percent byweight.
 4. A method according to claim 1, wherein the preform rodcomprises a concentration of TiO₂ of about 7.5 percent by weight.
 5. Amethod according to claim 1, wherein the preform rod is substantiallycolorless.
 6. A method of manufacturing a laser fiber comprising thesteps of:a) providing a preform rod consisting essentially of asubstantially colorless silica-titania glass having a concentration ofTiO₂ in a range of about 6 to 8 percent by weight; b) heating thepreform rod to a drawing temperature; c) pulling the rod to form anoptical fiber; and d) forming a tip on the fiber.
 7. A method accordingto claim 1, wherein the concentration of TiO₂ in the silica-titaniaglass is about 7.5 percent by weight.
 8. A method according to claim 6,further comprising a step of doping the fiber tip with a laser absorbingdopant.
 9. A method according to claim 8, wherein the step of dopingfurther comprises doping the fiber tip with a predeterminedconcentration of the dopant.